System and method for flexible solar tracker and testing

ABSTRACT

Disclosed herein is a technique of configuring flexible photovoltaic tracker systems with high damping and low angle stow positions. Under dynamic environmental loads implementing a high amount of damping (e.g., greater than 25% of critical damping, greater than 50% of critical damping) or a very high amount of damping (e.g., 100% or greater of critical damping, infinite damping) enables the flexible tracker system to prevent problematic aeroelastic behaviors while positioned in a low stow angle. The disclosed technique is further applied to a prototyping process during wind tunnel testing.

TECHNICAL FIELD

The present application is related to solar tracker systems for solarpanels.

BACKGROUND

Photovoltaic (PV) power systems frequently track the sun to variousdegrees to increase an amount of energy produced by the system. Thesetrackers typically move photovoltaic modules to adjust an angle ofincidence of the sunlight on the surface of the PV modules. Inparticular, trackers typically rotate the PV modules around an axisprincipally oriented north to south, tilting the modules to as much as60 degrees towards the east and west and adjusting tilt within thisrange throughout the day. By tracking the position of the sun, PV powersystems often produce 20-30% more energy than fixed-tilt systems.

A common configuration of horizontal single-axis trackers (“SAT”) asdescribed above includes a single actuator near the center of a row ofPV modules, potentially with 80-120 modules tilted by a single actuator.The angle of tilt is defined by the position of the actuator, while atorque tube or other similar device transfers moments and positions therest of the row at the tilt of the actuator. However, environmentalloading (wind, snow, dead load, etc.) can twist portions of a row awayfrom the intended tilt angle. These types of solar trackers are referredto as “flexible” within the industry in comparison to types that use anactuator on sufficient points along a solar tracker row to constrainmaximum twist to less than 10 degrees delta measured along a given row.Solar trackers that exhibit meaningful twisting under wind loadingrequire that both static and dynamic impacts be considered through windtunnel testing. The combination of static and dynamic wind loadingresults in a total system wind loading. The twisting is typical of othertypes of flexible structures that deform under wind loading and is wellstudied in the industry through aeroelastic wind tunnel testing andrelated simulation modeling.

The prevailing technique for mitigating environmental load is through ahigh angle stow position with minimal damping of the solar tracker. Ahigh angle stow position refers to positioning the panel more verticallythan horizontally. The high angle stow reduces the potential of highdynamic wind loading.

When a new PV system project is developed, the system is tested in awind tunnel to optimize the cost of components of the system as afunction of the projected output of the system. Wind tunnel tests areeither static or aeroelastic/dynamic.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic view of an installed solar tracker system asmay be employed in embodiments.

FIG. 2 illustrates a damper are positioned throughout the solar trackersystem.

FIG. 3 depicts an embodiment of a PV string tracker module.

FIGS. 4A-4C illustrate an example damper with a variable damping ratio.

FIG. 5 illustrates an example Durst curve.

FIG. 6 is a flowchart illustrating a method of conducting a wind tunneltests under high damping or very high damping.

The figures depict various embodiments of this disclosure for purposesof illustration only. One skilled in the art can readily recognize fromthe following discussion that alternative embodiments of the structuresand methods illustrated herein can be employed without departing fromthe principles of the invention described herein.

DETAILED DESCRIPTION

Modern photovoltaic (PV) systems make use of high stow angles and lowdamping (e.g., less than 25% of critical damping, 3-15% of criticaldamping). Critical damping refers to an amount of damping applied to agiven object such that there is no oscillation of the object (such as aPV panel) in response to a force on that object. Specifically, theobject moves only as far as the force pushes and no further (e.g., thesystem reaches equilibrium and force is dissipated without oscillation).

“Critical damping” is an effect that may be achieved on the level of thesolar tracker half-row (i.e. a “wing”). However, small sections of a fewpanels may oscillate far from the damper locations on such a row. Thesesmall oscillations do not drive the overall oscillation behavior of theentire wing and can be safely ignored.

For the purposes of this disclosure, “high damping” refers to more than25% of critical damping, and “very high damping” refers to 100% ofcritical damping or greater (e.g., being overdamped). High damping is arelative term based on the context upon which damping is applied. Knownprevailing systems do not use greater than 25% damping, thus dampinggreater than the prevailing systems is considered high. A system withhigh damping, but less than critical damping will eventually reachequilibrium, though will allow for oscillation. An overdamped systemreaches equilibrium, as does critical damping, but after a longer periodof time (and similarly without oscillation). “Infinite” damping refersto a fixed position.

“Partially locking” refers to dampers that have a variable damping ratioor stage change (i.e. a port that closes, but that does not hard lock).“Fully-locking” similarly refers to a damper with a stage change, but atleast one stage implements an infinite damping ratio.

Increasing damping on a PV panel has a number of side effects. First,the force applied to the remainder of the panel structure is increased.Because force is not being expended moving/oscillating, the remainingstructure must absorb the force. Examples of portions of the remainingstructure with increased stresses include the bracketry attaching thedamper to the torque tubes or rotating superstructure that supports themodules, any bolts that attach damper to brackets and brackets to torquetubes, localized wall stresses of the torque tubes themselves wherebracketry attaches (for example, where a U-bolt may crush a thin-walledtube), and foundations that dampers/brackets attach to.

Accordingly, the remaining parts of the structural system are designedmore robustly to withstand the shared environmental load. Notably, theforces on non-damping components such as the actuator itself are oftenreduced. Second, increasing damping on a given PV panel makes that panelmore difficult to intentionally rotate for tracking the sun.

Flexible PV systems generally rely on as few actuators as possible(actuators are comparatively expensive parts). The ratio of actuators topanels is higher than in non-flexible systems. Non-flexible systems usemore actuators in place of damping; however, the additional actuatorsincrease the overall cost of the system.

For purposes of this disclosure a “flexible” solar tracker system is onesubject to sufficient deflection as to require aeroelasticconsideration. 10 degrees of absolute twist is a typical cut-off forwhen a static wind tunnel test report may be used without specificaeroelastic testing added in. However, the selection of 10 degrees ofabsolute twist is subjective on the part of the wind tunnel testfacilities and allows for a buffer between when aeroelastic effectsbegin to dominate. Flexible tracker systems allow for deflectionrequiring aeroelastic consideration due to a relative lack of points offixity along each row. Actuators generally act as points of fixity. Rowsthat have few (or a single) actuator or other point of fixity perpanel/module are flexible.

In circumstances with permanent high damping, while the PV panelsnearest to the actuator will position correctly, those panels that arefurther away from the actuator will be skewed based on the differencesin torque at different distances from the actuator (via the torquetube). The skew will lead to panels shading adjacent panels and causinga decrease in efficiency.

Low damping (e.g., 10-15% of critical damping, less than 25%) is notsufficient to prevent torsional galloping or flutter on flexible solartracker systems stowed at a low tilt angle under a design wind load.Dynamic loads propagate along a torque tube and cause galloping eventsthat cause damage to the array. Thus, high angle stow (e.g., 60 degrees)is used to prevent galloping events. High angle stows may causeadditional stresses to the system under static loads. Stowing flat hasbeen the industry standard until recent years, since low angles are thebest way to combat key static loads at high wind speeds. However, recentadvancements to string length (e.g., the number of panels wired inseries) have led to problematic aeroelastic behaviors for low anglestowing under dynamic stresses. Long strings (e.g., greater than 20panels) are more flexible and more prone to torsional galloping andother effects.

Described herein are wind tunnel testing methods used in the design of atracker system that is highly damped (25% critical damping to just undercritical damping) or very highly damped (critical damping or greater)against rotation about a central axis when considering total systemdamping. Examples of total system damping factors include: componentdampers, aerodynamic damping, damping due to friction, damping due tomaterial strain, etc.

Common design practice to address dynamic loading is to perform windtunnel testing with panel angles set at a high tilt angle under designwind loading (ex: >20 degrees from horizontal) to largely mitigate thehysteresis effects of flow separation. However, operation at high tiltangles can increase other types of measured pressures and thus cost whenwind tunnel test results are applied to the design of a solar trackersystem. In particular, static wind tunnel testing of solar trackerstructures set at a high tilt will show significant increases inpressures measured. Thus, there is a trade-off inherent in optimizingwind stow tilt angles for flexible solar tracker structures whenconsidering both static and dynamic wind load testing.

Solar tracker systems developed to date show a response time of rotationon the order of is or faster. The time of rotation is a function of thenatural frequency (0.5˜1.5 hz for 1^(st) order row rotation) and lowtotal system damping. For this reason, the wind tunnel testing methodsemployed currently are easily separated into “dynamic” (aeroelasticmethods) and “static” (rigid model methods) of wind tunnel testing.These two very different types of testing are then combined to determinea total system wind loading that can be considered to be the combinationof a baseline static contribution and an additional dynamic wind loadingeffect. Up until the point of critical damping, total wind loading willnot be less than a static wind loading test evaluated on a fullydeformed structure and to a building code standard wind gust (forexample 3s gust from IBC/ASCE 7).

Wind tunnel testing of flexible solar trackers and their scale models ata low stow angle (ex: <20 degrees from horizontal) under a design windloading can show significant aeroelastic behaviors when solar trackersystems exhibiting typical damping ratios below 15% are evaluated. Thesebehaviors are commonly referred to as torsional galloping, stallflutter, divergence, and buffeting. The primary cause of torsionalgalloping or stall flutter behaviors is a hysteresis effect in flowattachment and separation that adds inertial energy to the rotatingpanel assembly under wind loading when trackers are wind tunnel testedat low stow angles. The low total damping common on these systems doesnot mitigate the energy gain on each cycle and thus aeroelasticbehaviors are observed to increase. The whole of aeroelastic windloading behaviors described is frequently referred to as “dynamic” windloading. Common design practice to address dynamic loading is to performwind tunnel testing with panel angles set at a high tilt angle underdesign wind loading (ex: >20 degrees from horizontal) to largelymitigate the hysteresis effects of flow separation. However, high tiltcan increase other types of measured pressures and thus system cost whenwind tunnel test results are applied to the design of a solar trackersystem.

Wind tunnel testing of highly damped solar tracker models (greater than25% or greater than 50% critical damping) makes it possible to mitigatethe majority of undesired aeroelastic behaviors when tracker systems arestowed at a low tilt angle under design wind loads. Undesiredaeroelastic behaviors are mitigated because the energy dissipated by thetotal system damping can equal or exceed the hysteresis effect of flowseparation experienced over a given oscillation. Meaningful reductionsin dynamic wind loading are measured with aeroelastic wind tunneltesting and analysis, up to approximately the point of critical totalsystem damping.

By increasing total system damping of a wind tunnel tested PV trackersystem model above critical, it is possible to dramatically lower the1^(st) order damped natural frequency of the system such that the timeto respond to a wind load is greater than the modeled building code windgust duration time. By doing so, aeroelastic wind tunnel testing methodsand related analysis will show that longer effective gust durations arerequired for the modeled system to achieve maximum deflections. As windtunnel testing in a boundary layer implicates a turbulence intensitythat matches that of the atmospheric boundary later, test model responserates greater than the shortest equivalent gust durations will result inlower deflections than for solar tracker system models with less thancritical total system damping. This approach will result in a total windloading (static+dynamic) than can be lower on critical system componentsthan if the system was evaluated solely with a static wind tunnel teston a fully deformed structure for a building code standard wind gustduration. Lower total wind loading on the modeled system will result inreduced costs and increased reliability of full-scale PV systeminstallations.

In some embodiments, the wind tunnel testing indicates at what thresholdof wind strength the PV tracker system enters the stow state.

FIG. 1 shows a schematic view of an installed solar tracker system asmay be employed in embodiments. A solar tracker system 100 may includeindividually-actuated PV string rows 125 installed in a predeterminedlocation and orientation relative to the sun. Each PV string row 125 mayinclude one or more PV panel strings 120. In some embodiments eachstring row 125 includes thirty PV panels 110 and four strings (e.g., fora total of 120 panels). For example, two or more PV panel strings 120may be combined in a row and mechanically connected to a drive system ofa corresponding PV string tracker module 130.

Furthermore, each PV string may include several PV panels 110electrically connected in series. Thus, a PV string tracker nodule 130may individually move a corresponding PV string row 125 to orient the PVpanels 110 of the PV strings 120 as needed for effective systemoperation. For example, the PV string tracker module 130 may point thePV panels 110 toward the sun to maximize PV electrical generation duringdaytime, or the tracker module 130 may move the PV panels 110 into astowed position, e.g., for nighttime or weather stowage. Accordingly,the tracker module 130 may require electrical power at all times,including daytime and nighttime.

In some embodiments, the tracker module 130 may be powered byforward-fed power 142 from a corresponding PV string, or the trackermodule 130 may be powered by back-fed power 141 delivered from a stationhub 150 of the solar tracker system 100. The forward-fed power 142 maycorrespond, for example, to a rated system power generated by thecombined PV string rows. For example, the rated system power may includepower supplied by the PV strings 120 to a power cable running betweenthe PV string rows 125 and the station hub 150. The tracker module 130depicted herein may include an actuator such as a rotational actuator.Other embodiments include linear actuators that may or may not beintegrated into damper components. The form the actuator takes variesfrom system to system.

Each PV string 125 may be electrically connected in parallel with theother PV strings, and thus, the rated system power may have a ratedsystem voltage corresponding to a rated voltage of the PV strings 125.In some embodiments, the actuator of the tracker module 130 is a 12-48VDC brushless motor that is powered off a small PV panel on the trackerrow and/or off an integrated battery backup installed 1× per row. Othervariations may tap off the string voltage.

FIG. 1 depicts a working system in the field. During wind tunnel testinga scale or full-size equivalent is tested. A solar test apparatus mayreplace a number of parts such as the panels 110 themselves. The purposeof wind tunnel testing is to identify the stresses on the system andthus a number of components are unnecessary for testing purposes.Testing apparatus includes an actuator of the tracker module 130,physical structure of the system 100 (e.g., a base, a torque tube, etc.. . . ) dampers (see FIG. 2), PV panels 110 or equivalents. In someembodiments, the actuator is modeled as a point of fixity on the testmodel. The test equipment may include equipment that measures the actualmoment that the actuator would see in an aeroelastic wind tunnel test.

In some embodiments, reference to PV modules or panels 110 also refer topanel equivalents or to components affixed or coupled to panels or panelequivalents. Tracker systems are often manufactured or marketedindependently from the panels 110 themselves. Wind tunnel tests areperformed on systems 100 that do not include PV panels, but contemplatethe inclusion of PV panels. References to components affixed or coupledto panels herein contemplate being affixed to brackets that affix to thepanels and not necessarily to the panels themselves. To wit, suchbrackets are components that are stress tested during wind tunneltesting. These brackets are not specifically depicted in the figuresbecause their appearance, implementation, and even existence thereofvaries from system to system.

A given tracker and PV module system may connect the modules directly tothe tracker without brackets. The direct connection is done by addingcost to the PV module frame to eliminate the bracket structure. Forexample, the PV module may be directly bolted to the torque tube with au-bolt.

As shown in FIG. 2, a set of dampers are positioned throughout the solartracker system. Along the string rows 125, a number of modules 110include dampers 200 paired therewith. The dampers 200 affix between abase 210 of a given panel 110 or set of panels 110 and a panel 110 orbrackets affixed thereto. The collection of PV modules 110 is rotatablyanchored to a base 210. The amount of electricity produced by eachphotovoltaic module can be a function of at least the angle of incidenceof light on the surface of the module, where more energy is capturedwhen light is perpendicular to the surface (i.e., a zero-degree angle ofincidence) than when light is incident at higher angles.

Increasing or decreasing the length of the linear actuator changes atilt angle of the collection of PV modules 110 with respect to the base210. Other types of actuators may be used in other embodiments. Forexample, the PV module collection 110 may be mounted on an axle and arotary actuator may drive the axle to rotate the collection of PVmodules 110 around an axis.

The damper 200 provides damping for the PV system 100, resistingmovement of the PV modules 110 relative to the base 210. Damping by thedamper 200 can mitigate dynamic wind loading or other vibrational loadsapplied to the PV system 100. Wind loading can induce motion in PVsystem 100, for example rotating the collection of PV modules 110 aroundthe base at a velocity multiple orders of magnitude higher than themotion induced by the actuator. Although the damper 200 is shown in FIG.1 as a component separate from the actuator for purposes ofillustration, the damper 200 may be incorporated into or positionedconcentric to the actuator.

The damper 200 has a variable damping ratio. A variable damping ratioavoids torsional issues resulting from overdamping such as skewed panelswithin the same string that shade one another. The damper 200 has atleast a first damping ratio under a first operating condition and asecond damping ratio under a second operating condition.

Different damping ratios are advantageous for different operatingstates. For example, a high damping ratio or very high damping ratioenables the damper 200 to dissipate more energy, and therefore bettermitigates undesired oscillations of the PV system 100 under wind loadingthan a low damping ratio. A high or very high damping ratio also enablesthe damper 200 to bear a portion of the static load of the PV modulecollection 110 and dynamic loads caused by environmental conditions,reducing the load on the actuator. The high or very high damping ratiois present while the system is under stress of a wind flow, or otherenvironmental stress exceeding a threshold value of force/speed.

However, a high damping ratio may cause the damper 200 to provide enoughresistance to the movement of the actuator to cause the PV module 110 totwist away from its intended orientation. As a result of the modifiedangle of incidence caused by this “propeller effect,” the collection ofPV modules 110 may generate less electricity. If twisted more than a fewdegrees, operation of the collection of PV modules 110 may fall outsideacceptable specifications.

A low damping ratio, in contrast, reduces the twist by providing lowerresistance to movement of the actuator. Accordingly, while not underthreshold environmental stresses, the damper 200 has a low dampingratio. Routine tracking behavior is performed while the damper 200operates at the low damping ratio. The total system damping ratio isrelatively low (e.g., sub-25% of critical damping) when the PV modules110 move at low speeds relative to the base 210 (e.g., while theactuator is moving the collection of PV modules 110 without highenvironmental loading) and relatively high when the PV modules 110 moveat higher speeds relative to the base (e.g., under dynamic windloading). The higher damping ratio of the damper 200 enables the damper200 to support a portion of the loading on the PV system 100, includingthe static load of the PV module collection (e.g., static wind loading)or dynamic loading caused by environmental conditions such as wind,snow, or dust. The lower damping ratio reduces the damper's resistanceto movement caused by the actuator. The damping ratio of the damper 200changes passively based on the operating state of the actuator, such asthe actuation rate. The damping ratio is therefore adjusted withoutactive control by, for example, the system controller.

As non-limiting examples of the operation of passive dampers, thedamping ratio may passively change between relatively low damping andrelatively high damping, or may passively change between relatively lowdamping and very high (or “partially-locking”) damping, or may passivelychange between relatively low damping and “infinite” (or“fully-locking”) damping, or may passively change between relativelyhigh damping and very high (high and very high damping are“partially-locking”) damping, or may passively change between highdamping and “infinite” (or “fully-locking”) damping, or may passivelychange between very high (or “partially-locking”) damping and “infinite”(or “fully-locking”) damping. As non-limiting examples of the operationof active dampers, the damping ratio may actively change betweenrelatively low damping and relatively high damping, or may activelychange between relatively low damping and high or very high (or“partially-locking”) damping, or may actively change between relativelylow damping and “infinite” (or “fully-locking”) damping, or may activelychange between relatively high damping and very high (consistently“partially-locking”) damping, or may actively change between relativelyhigh damping and “infinite” (or “fully-locking”) damping, or mayactively change between very high (or “partially-locking”) damping and“infinite” (or “fully-locking”) damping.

The high damping ratio has a value greater than 25% of critical damping,greater than 50% critical damping, or greater than 70% critical damping.A very high damping ratio has a value of 100% critical damping, greaterthan 100% critical damping, or infinite damping (such that the PV system100 is overdamped).

FIG. 3 depicts an embodiment of a PV string tracker module 130. thetracker module 130, panel actuator 220, PV string terminals 320,transmission mechanism 330, brake 240, pile/base 310, tracker controller250, drive 340, sensor 350, and sensor lines 351 and 352. Notably, whileactuator 220 is called out specifically, elements 330, 220, and 240 arecommonly referred to, collectively, as the actuator. In someembodiments, the transmission mechanism 330 is self-locking whileactuator 220 is not driving (like a worm gear) and the brake 240 is notrequired.

Drive 340 may be a transmission mechanism such as gears of a planetarygear set and may have other configurations as well. The sensor 350 maypreferably be a hall effects sensor but may also be a shunt or haveanother configuration. The tracker module 130 may orient thecorresponding PV strings as required for solar tracking and/or systemstowage. Accordingly, the tracker module may include the pile 310supporting a transmission mechanism 330, e.g., a slew drive, linkage, orother mechanical drive mechanism to convert mechanical power from thepanel actuator 220 into movement of the PV string. The panel actuator220 may be operatively coupled to the PV string through the transmissionmechanism to affect movement of the PV string. The panel actuator 220may be, for example, a linear or rotary actuator, such as an electricalDC motor, e.g., a DC stepper motor and may be sized for a 400 W or lessback-fed power supply. Other sizes may also be used and may be sized toaccommodate inverter capabilities from the system hub or centralinverter. Some embodiments make use of more powerful motors, e.g., 10kW.

The tracker module 130 may also include a brake 240 having a brakesolenoid to limit movement of the panel actuator and/or the transmissionmechanism. The panel actuator 220 may be operatively coupled to thetracker controller 250 in that the tracker controller 250 may provideelectrical power and/or electrical signals to drive the panel actuator220. Accordingly, the tracker controller 250 may serve as an electricalor control interface between the tracker module 130 and the othercomponents of the solar tracker system 100. The tracker controller 250may also be used in commissioning. Default safety modes may also beperformed by the tracker controller 250. For example, loss ofcommunications and fault sensing may trigger the controller 250 to placethe PV string in stow mode.

During nighttime the tracker controller 250 may be dormant as little ifany nighttime movement may be preferably performed. However, whennighttime movement is to be performed, the controller 250 may serve toenergize back-fed power through communications with the station hub 150or other component of the system 100. A “wake-up” message may be sent bya controller 250 to other controllers 250 over power lines or wirelesslyand may also be sent to other components of the solar tracker system 100over power lines or wirelessly. This movement may be choreographed andstaged to stow the system

The tracker controller 250 may include controller terminals 320electrically connected to conductors of the power cable at the powerjunction. Similarly, the tracker controller 250 may be electricallyconnected in parallel with the PV string. For example, the conductorsleading from the controller terminals to the power junction may also bejoined with respective PV string terminals of the PV string.

In some embodiments, every tracker controller of the solar trackersystem may receive power from the PV strings in parallel. That is,several PV strings in a PV string row may be connected in parallel witheach other and to the PV string terminals. Thus, even if one PV stringin the PV string row fails, the tracker controller 250 may still receivepower from the PV string terminals and/or the power junction. Thetracker controller 250 may be electrically connected in parallel withthe PV strings in numerous configurations, as described below.

FIGS. 4A-4C show one example damper 200 having a variable damping ratio.FIG. 4A is a bottom cutaway view of the damper 200, while FIGS. 4B-4Care a side cutaway view of the damper. The depicted damper includes adamper piston 410 that moves through fluid contained in a damper chamber405. The damper chamber 405 includes any fluid or mixture of fluidsbased on a desired variable damping ratio. Examples include, air, water,oil, or non-Newtonian fluids. The damper piston 410 includes two ports415 that, when open, allow fluid to flow between the damper piston 410and damper chamber 405. The ports 415 are shown in FIG. 4A as openingsin a bottom end of the damper piston, but the ports can be locatedanywhere in the damper piston.

In other embodiments a two-chamber design is implemented whereby onechamber is in front of the end of the piston 410 and the other chamberis the trailing edge of the piston 410. Valves are formed on a plungerdevice on the piston 410.

The two ports 415 can include at least one smaller diameter port 415Aand at least one larger diameter port 415B. The larger diameter port415A is governed by a pressure valve 220. When the pressure valve 220experiences high pressure the value seals and the fluids are forced onlythrough the small diameter port 415A.

When the damper piston 410 moves through the fluid at low speeds (e.g.,while the PV modules 110 are rotated at a low speed by the actuator220), the fluid flows freely through both the large diameter port 415Band the small diameter port 415A where there is comparatively littleresistance to the movement of the piston. Conversely, when the pressurevalve 220 is sealed, fluid only (or primarily) passes through the smalldiameter port 415A. The reduced port area reduces the speed at which thepiston 410, and therefore the damper 200 compresses.

In some embodiments, there are three or more varied damping ratios. Afirst low damping ratio corresponds to routine tracking behavior.Remaining damping ratios correspond to varied thresholds of forceapplied by environmental loading. Each progressive threshold of forcecorresponds to a greater damping ratio.

FIG. 4B illustrates an example of the piston 410 moving at a low speedthrough the fluid. As shown in FIG. 4B, the valve 420 is open and fluidcan pass through the larger diameter port 415B to flow into or out ofthe damper piston 410. At higher speeds, the valve 420 is pushed closedand the fluid is only (or primarily) forced through the smaller diameterport 415A. The resistance provided by the fluid flow through the smalldiameter port 415A increases the effective damping ratio of the damper200.

FIG. 4C illustrates an example of the piston 410 moving at a high speedthrough the fluid. As shown in FIG. 4C, the valve 420 is closed andfluid is only (or primarily) forced through the smaller diameter port415A to flow into or out of the damper piston 410. Other embodiments ofthe damper 200 are feasible. The disclosed damper operates based on apremise of passively controlled fluidways that vary in diameter as aresult of loads applied to the damper 200. For example, valves mayregulate fluid flow through multiple equally or differently sized portsin the damper piston. At lower speeds, the valves are open to allow thefluid to flow through several or all of the ports. At higher forcesapplied, the valves close the port and force the fluid to flow through asmaller number of ports.

In some embodiments, additional all ports/fluidways close at a thresholdapplied force. As another example, the damper 200 may include anon-Newtonian fluid that has lower viscosity at low piston speeds andhigher viscosity at high piston speeds.

In some embodiments, the PV system 100 is designed based on wind speedin the area where the system is installed. In particular, the PV system100 may be designed to withstand expected peak loads from the area'swind conditions following a protocol such as ASCE 7. Similarly, thetests performed in a wind tunnel are often selected based on theexpected environmental conditions at the installation site.

FIG. 5 illustrates an example Durst curve, which relates average windspeed to gust duration, that may be used in such protocols. As shown inFIG. 5, average wind speeds are higher for shorter measurements of gustduration than for longer measurements. Because the damper 200 has ahigher damping ratio under wind loading and bears a portion of the loadon the collection of PV modules 110, the PV system 100 may be designedbased on longer gust durations—and therefore lower wind speeds—thanphotovoltaic systems lacking the damper 200. Furthermore, while theDurst curve shown in FIG. 5 assumes free, unobstructed wind speed, thePV system 100 will likely experience turbulent air flow as dynamic windsmove around the structure. The average moments on the PV system 100under turbulent flow may be even lower across longer gust durations thanpredicted by the Durst curve. Accordingly, at least one of the bases210, the actuator 220, and the PV modules 110 are designed to withstandan average value of moments applied to the PV system 100 across aspecified duration of time. The duration of time can be calculated basedon wind tunnel testing, and can be, for example, approximatelyequivalent to a response time of the PV system 100 under targetenvironmental loads. The design for lower wind speeds may reduce theamount of material used to construct the base 210, the actuator 220, andthe collection of PV modules 110, and may reduce overhead andmaintenance costs for the PV system 100.

In some embodiments, the higher damping ratio of the damper 200 isdesigned under wind tunnel testing to achieve a specified response timeof the PV system 100 under high environmental loads. Because the higherdamping ratio resists movement of the actuator 220, it may take longerfor the actuator 220 to move the PV modules 110 to a specified angleunder the higher damping ratio than under the lower damping ratio. Thehigher damping ratio can be selected such that the movement of the PVmodules 110 through a designated angular distance (relative to the base210) will take a specified amount of time if the PV system 100 issubjected to a specified amount of wind loading that is enoughenvironmental loading to cause the damper 200 to transition to thehigher damping ratio. For example, the higher damping ratio can beselected under wind tunnel testing such that the actuator moves the PVmodules 110 thirty degrees relative to the base in 60 seconds while thePV system 100 is subjected to a specified amount of wind loading above athreshold wind speed. The higher damping ratio can be selected to allowfaster or slower movements of the PV modules 110, such as 10 seconds, 30seconds, or 120 seconds.

FIG. 6 is a flowchart illustrating a method of conducting a wind tunneltests under high damping or very high damping. In step 602, a prototypetracker system is arranged within a wind tunnel. The prototype trackersystem is of a flexible design, and thus including few actuators ascompared to the number of panels. In step 604, the value of total systemdamping is determined. Methods for determining total system dampinginclude performing pluck tests, computing numerically via physicsmodels, or conducting multiple preliminary rounds of wind tunneltesting.

In step 606, the prototype tracker system is equipped with high dampingor very high damping dampers. The number of dampers is based on thetotal number of panels, the weight and ancillary damping properties ofthe system, the damping provided by each damper, and the environmentalload for the given test as well as other factors.

In step 608, the prototype tracker system sets the available panels to alow stow angle (e.g., within 10 or 20 degrees of parallel to theground). In step 610, the wind tunnel executes a dynamic load test. Agiven test may result in acceptable aeroelastic behavior, orunacceptable aeroelastic behavior. In response to a test with acceptableaeroelastic behavior, in step 612, the component parts are evaluated forstresses. Components of the prototype tracker system are modifiedseeking optimization of the cost of the system based on estimatedoutput. The stress applied to the prototype is based on the amount ofdamping used in the given test.

In step 614, the prototyping endeavor tunes or modifies componentsand/or damping of the prototype system prioritizing reducing the overallcost of the system, while maintaining acceptable aeroelastic behaviorsduring the tests. Where high damping imputes more stress on theremaining system, more robust components are used to compensate. In somecircumstances more robust components cost more.

Total system damping ratio is a function of multiple variables thatchange across projects as well. Some of more significant variablesinclude natural frequency of the 1st mode of rotation, peak twist(amplitude), and changes in damping due to non-damper components (likesay how much bushing friction occurs or how much aerodynamic dampingoccurs). It is very likely that no two projects will have the exact samedamping. All of the above variability is considered by the bounds thatare placed on the system design by the dynamic wind tunnel test. In mostcircumstances, it is not possible to change a system parameter (e.g.,using a thicker walled torque tube) and not change total system damping.

In a given example, a tracker prototyping effort tunes the actual dampercomponent directly in response to a first dynamic test. That is, theymay test 50% total damping and choose to increase that value to 100% inresponse to a test round.

Other Considerations

The foregoing description of various embodiments of the claimed subjectmatter has been provided for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit the claimedsubject matter to the precise forms disclosed. Many modifications andvariations can be apparent to one skilled in the art. Embodiments werechosen and described in order to best describe the principles of theinvention and its practical applications, thereby enabling othersskilled in the relevant art to understand the claimed subject matter,the various embodiments, and the various modifications that are suitedto the particular uses contemplated.

While embodiments have been described in the context of fullyfunctioning computers and computer systems, those skilled in the art canappreciate that the various embodiments are capable of being distributedas a program product in a variety of forms, and that the disclosureapplies equally regardless of the particular type of machine orcomputer-readable media used to actually effect the distribution.

Although the above Detailed Description describes certain embodimentsand the best mode contemplated, no matter how detailed the above appearsin text, the embodiments can be practiced in many ways. Details of thesystems and methods can vary considerably in their implementationdetails, while still being encompassed by the specification. As notedabove, particular terminology used when describing certain features oraspects of various embodiments should not be taken to imply that theterminology is being redefined herein to be restricted to any specificcharacteristics, features, or aspects of the invention with which thatterminology is associated. In general, the terms used in the followingclaims should not be construed to limit the invention to the specificembodiments disclosed in the specification, unless those terms areexplicitly defined herein. Accordingly, the actual scope of theinvention encompasses not only the disclosed embodiments, but also allequivalent ways of practicing or implementing the embodiments under theclaims.

The language used in the specification has been principally selected forreadability and instructional purposes, and it cannot have been selectedto delineate or circumscribe the inventive subject matter. It istherefore intended that the scope of the invention be limited not bythis Detailed Description, but rather by any claims that issue on anapplication based hereon. Accordingly, the disclosure of variousembodiments is intended to be illustrative, but not limiting, of thescope of the embodiments, which is set forth in the following claims.

1. A flexible solar tracker system comprising: an actuator configured toset a position of a set of photovoltaic (PV) panels in a row, theactuator having a flexible configuration with respect to the set of PVpanels; dampers corresponding to the set of PV panels and having a totalsystem damping ratio against row rotation greater than 25% of thecritical damping when measured under peak design wind loading; and acontroller configured to cause the actuator to set a low angle stowposition of the set of PV panels.
 2. The system of claim 1, wherein thelow angle stow position is less than 20 degrees from horizontal plane.3. The system of claim 1, wherein total system damping ratio of theflexible solar tracker system is based on any of: a pluck test; apredictive model; or previous wind tunnel tests.
 4. The system of claim1, wherein the total system damping ratio is critically damped.
 5. Thesystem of claim 1, wherein total system damping ratio is overdamped. 6.The system of claim 1, wherein the flexible solar tracker systemincludes non-functional, substitute panels configured for wind tunneltesting.
 7. The system of claim 1, wherein the set of PV panels in therow contains multiple strings of PV panels.
 8. The system of claim 1,wherein the dampers have a variable damping ratio activated passively.9. The system of claim 8, wherein the variable damping ratio is based onan environmental load on the PV panels.
 10. A flexible solar trackersystem comprising: a single actuator configured to set a position of aset of photovoltaic (PV) panels in a row; dampers corresponding to theset of PV panels, the dampers configured result in the flexible solartracker system having a total system damping ratio against row rotationwhen measured under peak design wind loading that is overdamped; and acontroller configured to cause the actuator to set a low angle stowposition of the set of PV panels.
 11. The system of claim 10, whereinthe low angle stow position is less than 20 degrees from horizontalplane.
 12. The system of claim 10, wherein total system damping ratio ofthe flexible solar tracker system is based on any of: a pluck test; apredictive model; or previous wind tunnel tests.
 13. The system of claim10, wherein the flexible solar tracker system includes on-functional,substitute panels configured for wind tunnel testing.
 14. The system ofclaim 10, wherein the set of PV panels in the row contains multiplestrings of PV panels.
 15. The system of claim 10, wherein the dampershave a variable damping ratio activated passively.
 16. The system ofclaim 15, wherein the variable damping ratio is based on anenvironmental load on the PV panels.
 17. A method of configuring aflexible solar tracker system comprising configuring an actuator to seta position of a set of photovoltaic (PV) panels in a row, the actuatorhaving a flexible configuration with respect to the set of PV panels;configuring dampers corresponding to the set of PV panels to have atotal system damping ratio against row rotation greater than 25% of thecritical damping when measured under peak design wind loading; andconfiguring a controller to cause the actuator to set a low angle stowposition of the set of PV panels.
 18. The method of claim 17, whereinthe total system damping ratio is critically damped.
 19. The method ofclaim 17, wherein total system damping ratio is overdamped.
 20. Themethod of claim 17, further comprising: passively varying a dampingratio of the dampers.
 21. The method of claim 20, wherein said passivevarying is based on an environmental load on the PV panels.